The present invention is related to freight brake control valve devices and in particular to the cooperative arrangement between the graduating valve and slide valve of the service piston that provides the service brake control functions of the control valve device.
The well-known ABD and ABDW Control Valve devices manufactured by the Westinghouse Air Brake Co. employ a graduating valve that is carried fast with the service piston and a slide valve that is carried by the service piston through a lost-motion arrangement, so that relative movement can occur between the graduating valve and slide valve when the service piston is initially actuated from a release position to an application position and vice-versa. This relative movement between the graduating valve and slide valve provides for valve operation through the sliding engagement between adjoining lap fit faces thereof.
For example, in application position, an application passage in the slide valve is connected to the control valve brake cylinder passage at the slide valve/bushing interface, and the back end of the graduating valve uncovers a feed port of this application passage at the graduating valve/slide valve interface, to thereby supply auxiliary reservoir air effective on one side of the service piston to the brake cylinder When the auxiliary reservoir air pressure is accordingly reduced to approximately the pressure of the brake pipe air acting on the opposite side of the service piston, a spring forces the service piston back toward release position. During this initial release movement, however, only the graduating valve moves with the service piston, due to the lost-motion connection between the service piston and slide valve. Consequently, the uncovered feed port at the slide valve is lapped or covered by the graduating valve to terminate the supply of air to the brake cylinder. When this occurs, the service piston is in a lap position and no additional increase or decrease of brake cylinder pressure should occur.
It is known, however, that when the brakes are lapped for an extended period of time, such as when a train is descending a long grade, for example, a slight leakage at the graduating valve can cause the brake cylinder pressure to gradually creep up, so that a higher brake pressure than intended develops.
While leakage does not occur during the expected normal operation of the valve, the sequencing of port opening and closing by the graduating valve as it moves to service lap position does not positively prevent this "leak-up" of brake cylinder pressure.
The undesired buildup of brake cylinder pressure in service lap can conceivably occur in a valve having an abnormally high resistance due to the fact that another passage in the slide valve, known as the stabilizing passage, which is intended to be uncovered by the graduating valve in this lap position, in fact opens just prior to the application passage becoming lapped. This stabilizing passage establishes communication between the one side of the service piston that is subject to auxiliary reservoir air and the opposite side of the piston that is subject to brake pipe air, in order to dampen out spurious pressure variations therebetween. However, in opening the stabilizing passage prior to closure of the application passage, it is possible that both of these passages can be cracked open at the same time, in a certain position of the service piston. This allows auxiliary reservoir air to leak to the brake cylinder. Normally this would result in a reduction of auxiliary reservoir pressure on the one side of the service piston relative to brake pipe pressure on the opposite side to cause the service piston to move further toward release and positively lap off the feed port of the application passage. However, since the stabilizing passage is also cracked open, brake pipe pressure maintains the leakage of auxiliary reservoir air to the brake cylinder, so that no reduction of auxiliary reservoir pressure can occur. Therefore, the service piston can remain stalled in this position, allowing auxiliary reservoir air to continue to leak to the brake cylinder.
The object of the invention is to reverse the valve sequence by which the stability port and feed port connections are controlled, in order to better assure a positive cut off of the brake cylinder supply in service lap position.
Another object of the invention is to achieve the foregoing objective in a simple, low-cost manner.
Briefly, these objectives are achieved by providing a control valve device comprising a piston member subject on one side to the fluid under pressure effective in a brake pipe and on the opposite side to the fluid under pressure in an auxiliary reservoir charged to the pressure carried in the brake pipe, a tailpiece projecting from the opposite side of the piston member and having a face in which a recess is formed, a graduating valve carried fast in the recess so as to be movable axially with the piston member and having a first planar surface, a slide valve carried by the piston member with relative movement therebetween, the slide valve having a second planar surface slidably engaged with the first planar surface, a first passage in the slide valve via which fluid under pressure is supplied from said opposite side of said piston member to said brake cylinder device when the piston member is actuated from a release position to an application position, the first passage having an opening in the second planar surface that is uncovered by the graduating valve in application position, and a second passage in the slide valve via which fluid pressure communication is established between the brake pipe and the other side of the piston member, the second passage having an opening in the second planar surface axially displaced from the opening of the first passage, the first planar surface having an axial dimension at least as great as the maximum axial distance between the openings of the first and second passages, such that the opening of the first passage is covered by the graduating valve when the opening of the second passage is uncovered by the graduating valve in a lap position intermediate the application and release positions.